class ExceptionListener

ExceptionListenerTest.php in drupal/core/vendor/symfony/http-kernel/Symfony/Component/HttpKernel/Tests/EventListener/ExceptionListenerTest.php

core.services.yml in drupal/core/core.services.yml

drupal/core/core.services.yml

exception_listener in drupal/core/core.services.yml

Symfony\Component\HttpKernel\EventListener\ExceptionListener

class ExceptionListener implements EventSubscriberInterface {
  protected $controller;
  protected $logger;
  public function __construct($controller, LoggerInterface $logger = null) {
    $this->controller = $controller;
    $this->logger = $logger;
  }
  public function onKernelException(GetResponseForExceptionEvent $event) {
    static $handling;
    if (true === $handling) {
      return false;
    }
    $handling = true;
    $exception = $event
      ->getException();
    $request = $event
      ->getRequest();
    $this
      ->logException($exception, sprintf('Uncaught PHP Exception %s: "%s" at %s line %s', get_class($exception), $exception
      ->getMessage(), $exception
      ->getFile(), $exception
      ->getLine()));
    $attributes = array(
      '_controller' => $this->controller,
      'exception' => FlattenException::create($exception),
      'logger' => $this->logger instanceof DebugLoggerInterface ? $this->logger : null,
      'format' => $request
        ->getRequestFormat(),
    );
    $request = $request
      ->duplicate(null, null, $attributes);
    $request
      ->setMethod('GET');
    try {
      $response = $event
        ->getKernel()
        ->handle($request, HttpKernelInterface::SUB_REQUEST, true);
    } catch (\Exception $e) {
      $this
        ->logException($exception, sprintf('Exception thrown when handling an exception (%s: %s)', get_class($e), $e
        ->getMessage()), false);

      // set handling to false otherwise it wont be able to handle further more
      $handling = false;

      // re-throw the exception from within HttpKernel as this is a catch-all
      return;
    }
    $event
      ->setResponse($response);
    $handling = false;
  }
  public static function getSubscribedEvents() {
    return array(
      KernelEvents::EXCEPTION => array(
        'onKernelException',
        -128,
      ),
    );
  }

  /**
   * Logs an exception.
   *
   * @param \Exception $exception The original \Exception instance
   * @param string     $message   The error message to log
   * @param Boolean    $original  False when the handling of the exception thrown another exception
   */
  protected function logException(\Exception $exception, $message, $original = true) {
    $isCritical = !$exception instanceof HttpExceptionInterface || $exception
      ->getStatusCode() >= 500;
    $context = array(
      'exception' => $exception,
    );
    if (null !== $this->logger) {
      if ($isCritical) {
        $this->logger
          ->critical($message, $context);
      }
      else {
        $this->logger
          ->error($message, $context);
      }
    }
    elseif (!$original || $isCritical) {
      error_log($message);
    }
  }

}